Guide bushes mounted on the column of an automatic lathe to hold a rodlike workpiece for rotation at a position near a cutting tool are classified into rotary guide bushes and stationary guide bushes. A rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding. A stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding.
A guide bush of either type has a portion having a taper outer surface provided with slits to make the same portion elastic, a threaded portion to hold the guide bush on the column, and an inner surface for holding a workpiece. The inner surface always in sliding contact with a workpiece is liable to be worn and, particularly, the inner surface of a stationary guide bush is worn rapidly.
A guide bush proposed in JP-A No. 4-141303 has an inner surface to be in sliding contact with a workpiece, which slides and rotates on the inner surface, attached with a superhard alloy or a ceramic material attached to the inner surface by brazing or the like.
When the inner surface of a guide bush is attached with a superhard alloy or a ceramic material excellent in wear resistance and heat resistance, the wear of the inner surface of the guide bush can be reduced to some extent.
However, when the workpiece is subjected to heavy machining on an automatic lathe, in which the depth of cut is large and the cutting speed is high, the workpiece is damaged or seizing occurs due to decrease in the diametrical clearance between the guide bush and the workpiece even if the inner surface of the guide bush is attached with a superhard alloy or a ceramic material, because the superhard alloy and the ceramic material have a comparatively large coefficient of friction and a low thermal conductivity. Therefore, it has been difficult to increase the depth of cut and cutting speed.
The stationary guide bush has advantages that a workpiece can be accurately machined in a high roundness because the workpiece can be held so that its axis may not run out, less noise is generated, and the automatic lathe may be of a simple, compact construction.
However, the inner surface of the stationary guide bush is worn far more rapidly than that of the rotary guide bush and hence it is more difficult to increase depth of cut and cutting speed when the stationary guide bush is employed.
Further, in the automatic lathe, there frequently occurs a case where a part of the workpiece which has been once cut is drawn again in the guide bush and held, and it is subjected to machining again. In such a case, since small projections which are so-called flashes are frequently produced at the edge of the part of the workpiece, there is a problem that a flaw is produced at a portion adjacent to the edge of the inner surface of the bore of the guide bush and on the inner surface by the flash when the cut part is drawn in the guide bush. The flashes are frequently produced in the case where the workpiece is made of a material having high toughness which is hardly cut, which makes the aforementioned problem serious.
Accordingly, it is an object of the present invention to solve such problems so as to improve wear resistance of an inner surface of a guide bush to be in contact with a workpiece, and capable of enabling an automatic lathe to machine a workpiece at an increased depth of cut and an increased cutting speed without damaging the workpiece or causing seizure between the guide bush and the workpiece, and further, even when the part of the workpiece which has been cut is drawn into the guide bush so as to be subjected to machining again, the same guide bush can be made to be employed over a long period of time, eliminating the problem where a flaw is produced at a portion adjacent to the edge of the inner surface of the bore of the guide bush and on the inner surface by the flash.